The invention relates generally to providing an apparatus for electropolishing products made from metals. More particularly, the invention relates to an apparatus for electropolishing medical devices made of titanium, stainless steel, tungsten, nickel-titanium, tantalum, cobalt-chromium-tungsten, or cobalt-chromium.
While a wide range of products or devices can be made from the listed metal alloys for use with the present invention, medical devices are particularly suitable due to the biocompatible characteristics of these alloys. Thus, for example, implantable medical devices or devices that are used within the human body are particularly suitable and can be made from these alloys that have been electropolished in accordance with the present invention. More particularly, and as described in more detail herein, intravascular stents can be made from the listed alloys that have been electropolished according to the invention. Thus, while the description of prior art devices and of the invention herein refers mainly to intravascular stents, the invention is not so limited to medical products or intravascular stents.
Stents are generally metallic tube shaped intravascular devices which are placed within a blood vessel to structurally hold open the vessel. The device can be used to maintain the patency of a blood vessel immediately after intravascular treatments and can be used to reduce the likelihood of development of restenosis. Expandable stents are frequently used as they may travel in compressed form to the stenotic site generally either crimped onto an inflation balloon or compressed into a containment sheath in a known manner.
Metal stents can be formed in a variety of expandable configurations such as helically wound wire stents, wire mesh stents, weaved wire stents, metallic serpentine stents, or in the form of a chain of corrugated rings. Expandable stents, such as wire mesh, serpentine, and corrugated ring designs, for example, do not possess uniformly solid tubular walls. Although generally cylindrical in overall shape, the walls of such stents are perforated often in a framework design of wire-like elements or struts connected together or in a weave design of cross threaded wire.
Expandable stents formed from metal offer a number of advantages and are widely used. Metallic serpentine stents, for example, not only provide strength and rigidity once implanted they also are designed sufficiently compressible and flexible for traveling through the tortuous pathways of the vessel route prior to arrival at the stenotic site. Additionally, metallic stents may be radiopaque, thus easily visible by radiation illumination techniques such as x-ray film.
It is highly desirable for the surface of the stent to be extremely smooth so that it can be inserted easily and experience low-friction travel through the tortuous vessel pathway prior to implantation. A roughened outer surface may result in increased frictional obstruction during insertion and excess drag during travel to the stenotic site as well as damaging the endothelium lining of the vessel wall. A rough surface may cause frictional resistence to such an extent as to prevent travel to desired distal locations. A rough finish may also cause damage to the underlying inflation balloon. A less rough finish decreases thrombogenicity and increases corrosion resistance.
Stents have been formed from various metals including stainless steel, tantalum, titanium, tungsten, nickel-titanium which is commonly called Nitinol, and alloys formed with cobalt and chromium. Stainless steel has been extensively used to form stents and has often been the material of choice for stent construction. Stainless steel is corrosion resistant, strong, yet may be cut into very thin-walled stent patterns.
Cobalt-chromium alloy is a metal that has proven advantages when used in stent applications. Stents made from a cobalt-chromium alloy may be thinner and lighter in weight than stents made from other metallic materials, including stainless steel. Cobalt-chromium alloy is also a denser metal than stainless steel. Additionally, cobalt-chromium stents are nontranslucent to certain electromagnetic radiation waves, such as x-rays, and, relative to stainless steel stents, provide a higher degree of radiopacity, thus being easier to identify in the body under fluoroscopy.
Metal stents, however, suffer from a number of disadvantages. They often require processing to eliminate undesirable burrs, nicks, or sharp ends. Expandable metal stents are frequently formed by use of a laser to cut a framework design from a tube of metal. The tubular stent wall is formed into a lattice arrangement consisting of metal struts with gaps there between. Laser cutting, however, typically is at high temperature and often leaves debris and slag material attached to the stent. Such material, if left on a stent, would render the stent unacceptable for implantation. Treatment to remove the slag, burrs, and nicks is therefore required to provide a device suitable for use in a body lumen.
Descaling is a first treatment of the surface in preparation for further surface treatment such as electropolishing. Descaling may include, for example, scraping the stent with a diamond file, followed by dipping the stent in a hydrochloric acid or an HCl mixture, and thereafter cleaning the stent ultrasonically. A successfully descaled metal stent should be substantially slag-free in preparation for subsequent electropolishing.
Further finishing is often accomplished by the well known technique of electropolishing. Grinding, vibration, and tumbling techniques are often not suited to be employed on small detailed parts such as stents.
Electropolishing is an electrochemical process by which surface metal is dissolved. Sometimes referred to as xe2x80x9creverse plating,xe2x80x9d the electropolishing process actually removes metal from the surface desired to be smoothed. The metal stent is connected to a power supply (the anode) and is immersed in a liquid electrolytic solution along with a metal cathode connected to the negative terminal of the power supply. Current is applied and flows from the stent, causing it to become polarized. The applied current controls the rate at which the metal ions of the anodic stent are generally removed and diffused through the solution to the cathode.
The rate of the electrochemical reaction is proportional to the current density. The positioning and thickness of the cathode in relation to the stent is important to make available an even distribution of current to the desired portion of the stent sought to be smoothed. For example, some prior art devices have a cathode in the form of a flat plate or a triangular or single wire loop configuration, which may not yield a stent or other medical device with a smooth surface on all exposed surfaces. For example, the prior art devices do not always provide a stent having a smooth surface on the inner tubular wall of the stent where blood flow will pass.
The straightforward application of current, however, does not necessarily translate to even distribution of current across the entire surface sought to be polished. One important feature to creating an even surface on the desired portion of the part is the formation of current differential during the electropolishing process across the surface. Electropolishing provides varied current density to the surface imperfections such as undulations creating protrusions and valleys on the surface. Current density is highest at high points on the surface and lowest at the low points. The increased current density at the raised points causes the metal to dissolve faster at these points thus leveling the surface while forming a corrosion-inhibiting oxide layer.
Electropolishing in the proper electrolytic solution, can serve to smooth out the exposed rough surface to the point where it is ultrasmooth, shiny, and reflective. However, heretofore there has been no effective method to consistently produce an ultrasmooth, shiny finish on the surface a stent comprised of any metal or metal alloy such as cobalt-chromium alloys.
Treated with traditional electrolytic or etching solutions, such as those specified in ASTM E407-93, ASTM340-95, and ASTM E1558-93, a stent formed of metal alloy may exhibit a variety of finishes that may include a rather rough and unfinished appearance, a matte finish which is pitted, brown, blackened, feathered, etched, dimpled, rough, and/or uneven.
What is needed is an apparatus and a process for treating a product or device made of a metal alloy to consistently produce an ultrasmooth surface. The present invention satisfies this need.
The invention is directed to an apparatus and a process of electropolishing a product or device made from a metal alloy. More particularly, the product or device is suitably made from an alloy of stainless steel, titanium, nickeltitanium (Nitinol), tungsten, tantalum, cobalt-chromium-tungsten, or cobalt-chromium and is particularly useful for medical devices such as medical implants, hipjoints, bone screws, guide wires, catheters, filters, and intravascular stents. Other products and devices unrelated to the medical device products described herein also will benefit from the electropolishing apparatus and process when such products or devices are made from the listed metal alloys. Since the electropolishing apparatus and process of the invention are particularly useful for medical devices, and more particularly useful for intravascular stents, the process is described herein with respect to stents, but is not so limited.
The invention is directed to an improved product or device, such as a stent, formed from a metal alloy, that possesses an ultrasmooth shiny exterior surface. This invention is also directed to an apparatus for electropolishing such a stent using an acidic electrolytic solution to produce an exceptionally smooth surface. This invention also contemplates use of a composition of the electrolytic solution that serves to produce the ultrasmooth surface.
In keeping with the invention, an apparatus is provided, along with a process, for electropolishing a device made from a metal alloy. More particularly, the electropolishing apparatus includes a cathode and an anode submerged in an electrolytic solution for the purpose of removing metal from the surface of the metal alloy. The cathode design is intended to substantially surround the anode, which is the device made from the metal alloy, to remove metal from all exposed surfaces.
In one embodiment, a wire coiled in a spiral or helix configuration is submerged in a container of an electrolytic solution. The device, for example, an intravascular stent is the anode and is positioned within the coil and a current is applied from the anode, polarizing it, and thus encouraging metal ions of the stent to diffuse through the solution to the cathode. As the metal ions from the stent continue to diffuse, the surface irregularities diminish such that, after a predetermined time period, the surface of the stent becomes ultrasmooth and shiny. Importantly, all of the exposed surface area of the stent, including the outer and inner surfaces of the stent are polished smoother than a stent polished using conventional methods of electro-polishing. It is important for the outer tubular surface and the inner tubular surface of the stent to be smooth, since the outer surface contacts the vessel or arterial wall, and the inner surface is in contact with the blood flowing through the vessel or artery.
In another embodiment, a cylindrical tube formed of a mesh or weave configuration is provided as the cathode which is subsequently submerged or immersed in an electrolytic solution. As previously described, the stent, which is the anode, is then positioned within the cylindrical wire mesh tube for purposes of electropolishing the stent. A current is applied and flows from the anode (stent), polarizing it, and thus encouraging the metal ions of the stent to diffuse through the solution to the wire mesh tube cathode. After a predetermined time, the entire outer surface of the stent becomes ultrasmooth and shiny.